Table 1.

Characteristics of included studies

Reference	Sample characteristics	Strain	Study design	Duration of intervention	Measurement	Key findings and conclusions
Akkasheh et al. [34]	40 MDD patients. Ages 20–55 years	Lactobacillus acidophilus, L. casei, and Bifidobacterium bifidum	Double-blind, randomized, placebo-controlled trial	8 weeks	BDI	Consumption of probiotic supplement improved BDI scores
Benton et al. [30]	124 healthy humans. Avg. age: 62 years	L. casei	Double-blind, randomized, placebo-controlled trial	3 weeks	POMS, self-rated mood	No effect of probiotic on POMS results. Consumption of probiotic-containing yogurt improved self-reported mood of those whose mood was initially poor
Chung et al. [32]	36 healthy humans. Ages 60–75 years	L. helveticus	Double-blind, randomized, placebo-controlled trial	12 weeks	PSS, GDS-SF, DST, SRT, VLT, RVIP, Stroop Task	No significant effects of probiotics on the PSS, GDS-SF. Consumption of probiotics did improve DST, SRT, VLT, RVIP, and stroop tasks scores
Gruenwald et al. [36]	34 adults suffering from stress or exhaustion. Mean age: 44 years	L. acidophilus and B. bifidum and longum	Pre- and post-intervention assessment	6 months	PNQ, EWL	Subjects’ general condition improved by 40.7%. 73% of participants rated the effect of treatment as “good” or “very good”
Hilimire et al. [38]	710 young adults. Mean age: 19 years	Unknown	Self-report questionnaires on fermented food consumption, neuroticism and social anxiety	N/A	BFI, SPAI-23	Consumption of fermented foods containing probiotics was negatively associated with symptoms of social anxiety and interacts with neuroticism to predict social anxiety symptoms. Those at higher genetic risk for social anxiety disorder (indexed by high neuroticism) show fewer social anxiety symptoms when they consume more fermented foods
Marcos et al. [37]	136 healthy students. Ages 18–23 years	L. casei	Prospective, randomized, controlled, parallel study	6 weeks	STAI	No significant effects of probiotics on anxiety levels. Probiotics did modulate

MDD major depressive disorder, BDI Beck Depression Inventory, POMS profile of mood states scale, PSS perceived stress scale, GDS-SF geriatric depression scale, DST digit span test, SRT story recall test, VLT verbal learning test, RVIP rapid visual information-processing, PNQ psychological-neurologic questionnaire, EWL list of adjectives, bfi big five inventory, SPAI-23 social phobia and anxiety inventory, STAI state-trait anxiety inventory, HADS Hospital Anxiety Depression Scale, HSCL-90 Hopkins symptom checklist, CCL coping checklist, UFC urinary free cortisol, BAI Beck Anxiety Inventory, LEIDS-r Leiden index of depression sensitivity, fMRI functional magnetic resonance imaging